Alternative to IEnumerable<T>.Skip(1).Take(1).Single()

Question

I am having a difficult time with a seemingly easy and embarrassing problem. All I want is the next element in an IEnumberable without using Skip(1).Take(1).Single(). This example illustrates the basic problem.

private char _nextChar;
private IEnumerable<char> getAlphabet()
{
    yield return 'A';
    yield return 'B';
    yield return 'C';
}
public void sortAlphabet()
{
     foreach (char alpha in getAlphabet())
     {
         switch (alpha)
         {
             case 'A':  //When A pops up, I want to get the next element, ie 'B'
                 _nextChar = getAlphabet().Skip(1).Take(1).Single();
                 break;
             case 'B': //When B pops up, I want 'C' etc
                 _nextChar = getAlphabet().Skip(1).Take(1).Single();
                 break;
         }
     }
}

Other than being ugly, this example works. But let's say that the IEnumerable contained 2 million elements, then the LINQ statement makes the program execute unbearably slow. What I want is simple. I just want the next element in an IEnumberable<>. All my problems would be solved if there was a function like:

_nextChar = getAlphabet().moveNext() //or getNext()

It is much preferred if the solution keeps the same structure/layout/functionality of the example however, I am flexible. My program is a file parser, and among the 2 million lines of text are some keys like "money=324" where "money" and "324" are neighbor elements in the IEnumberable and when the parser comes across "money" I want "324". (who doesn't? :D Sorry for bad pun.)

Answer 1

All my problems would be solved if there was a function like:

_nextChar = getAlphabet().moveNext() //or getNext()

There is a function exactly like that. It just belongs to IEnumerator<T>, not IEnumerable<T>!

private char _nextChar;
private IEnumerable<char> getAlphabet()
{
    yield return 'A';
    yield return 'B';
    yield return 'C';
}

public void sortAlphabet()
{
    using (var enumerator = getAlphabet().GetEnumerator())
    {
        while (enumerator.MoveNext())
        {
            char alpha = enumerator.Current;
            switch (alpha)
            {
                case 'A':
                    if (enumerator.MoveNext())
                    {
                        _nextChar = enumerator.Currrent;
                    }
                    else
                    {
                        // You decide what to do in this case.
                    }
                    break;
                case 'B':
                    // etc.
                    break;
            }
        }
    }
}

Here's a question for you, though. Is it necessary that this code use an IEnumerable<char>, rather than an IList<char>? I ask because, as if this weren't obvious, the code would be much simpler if you had random access to the items returned by getAlphabet by index (and if someone is tempted to point out that you can do this with ElementAt, please, just get that idea out of your head right now).

I mean, consider what the code would look like in this case:

private char _nextChar;
private IList<char> getAlphabet()
{
    return Array.AsReadOnly(new[] { 'A', 'B', 'C' });
}

public void sortAlphabet()
{
    IList<char> alphabet = getAlphabet();
    for (int i = 0; i < alphabet.Count - 1; ++i)
    {
        char alpha = alphabet[i];
        switch (alpha)
        {
            case 'A':
                _nextChar = alphabet[i + 1];
                break;
            case 'B':
                // etc.
                break;
        }
    }
}

Isn't that much easier?